This application relates to an apparatus and method for assessing subgrade corrosion and, more particularly, to an apparatus and its use in determining corrosion at a specified site.
The power industry has been installing structures in direct contact with soil for more than a century and many of these structures have become unstable due to corrosion damage on their foundations. Inspection of these structures requires extensive equipment and manpower to first stabilize the structure and then excavate for a direct assessment. The issue is that many structures are buried to a depth of ten feet or more and have significant side loads which may cause them to topple without proper precautions. The volume of soil to be removed becomes extensive and open holes must be controlled to prevent accidents to personnel and the general public. Inspection crews can then consume massive amounts of time documenting the severity of damage and location in respect to groundline and usually utilities opt for repairs while the excavation is still open. This process limits productivity, becomes extremely expensive and consumes operational budgets due to inefficiencies.
Traditionally utilities estimated corrosion rates by periodically measuring corrosion pitting on metal surfaces. This approach provides a crude estimation of corrosion rate and does not discriminate between the controls of the corrosion cell. In an effort to reduce these costs, contractors and utilities developed models relying on environmental factors to describe soil corrosivity. Unfortunately, the resulting accuracy estimates ranged between 50 and 78%. It was realized through laboratory testing that soil properties vary greatly and those models require development for specific geographic locations. This dictates that the process must be repeated randomly throughout the service territory and then validated through excavation and direct assessment.
Many factors have been found to govern and influence corrosion rates found on transmission structures. Some factors originate from the operation of the circuits and some result from utilities that share the same Rights of Way. Pipelines, chemical plants, mining operations and railroads are high on the list of commercial operations that impact the corrosion rates, but internal factors include circulating currents, unbalanced transformers and long line effects due to the shield wire.
Environmental factors that may describe corrosion rates are moisture, temperature, pH, soil resistivity and the oxidation state of the structure. Dissimilarities in moisture, aeration and temperature are also known factors that may accelerate the corrosion process.
As a result of these different stresses, several types of corrosion affect transmission structures and other components. These include:                General corrosion (uniform)        Pitting/crevice corrosion (localized)        Galvanic        Concentration cell (differential oxygen or moisture)        Metal ion cell        Fatigue        Microbial        Long line effects        Stray current (AC or DC)        
Corrosion is an electrochemical process in which a transfer of electrons occurs during a chemical reaction. Electrochemical corrosion requires two processes to occur simultaneously, oxidation and reduction reactions. The oxidation reaction results in the liberation of electrons at the anodic site where the metal is corroding; the reduction reaction strips the electrons from the surfaces at the cathodic sites. The electrons in this circuit travel through the metal to the cathodic sites where they are consumed by electron acceptors (such as hydrogen ions to form a hydrogen atom). Charged ions migrate toward their respective sites in what is termed ionic conduction. There are anions with a negative charge and cations with a positive charge; the cations are categorized as electron acceptors while the anions are categorized as electron donors. See FIG. 1.
The ionic conduction to each site completes the circuit allowing corrosion to occur. The potential difference (voltage) between the two reaction sites is a measure of the driving force of the corrosion process. The current (amperage) between the sites is a measure of the rate at which the reactions are proceeding and may be considered the governing factor in the consumption rate of the material.
Common metals are typically found in nature as chemical compounds coupled with various oxides, chlorides, or sulfides. They seldom occur as pure metals. After they have been refined to an almost pure metal by man, nature wants to change them back to their original state in a process known as corrosion.